Method and apparatus for analysing a signal from a movement detector for determining if movement has been detected in an area under surveillance and an anti-theft system

ABSTRACT

An anti-theft system ( 1 ) for a motor vehicle detects movement in the motor vehicle, while at the same time avoiding false alarms. A movement detector ( 2 ) comprises a transmitter circuit ( 3 ) which transmits a microwave signal in bursts of around six nanoseconds duration and a receiver circuit ( 4 ) which receives the transmitted signals and reflected signals. The transmitted and reflected signals are mixed, and envelope detected in an envelope detector ( 22 ). The signals from the envelope detector ( 22 ) are passed through three filters, namely, a low pass filter ( 25 ) which passes low frequency signals in the range of 1 Hz to 40 Hz, a high pass filter ( 26 ) which passes high frequency signals of greater than 100 Hz and a band pass filter  15  ( 27 ) which passes intermediate frequency signals in the range of 40 Hz to 80 Hz. The signals from the filters ( 25,26,27 ) are analysed in a microprocessor ( 28 ). Slow movements in the vehicle are determined from the signals from the low pass filter ( 25 ), while fast movements in the vehicle are determined from the signal from the band pass filter ( 27 ). Interference signals, such as mobile phone signals are determined and identified from the signal from the high pass filter ( 26 ) and the low pass filter ( 25 ). In the event of interference being caused by a mobile phone, a low frequency energy alarm threshold value ( 40 ) of the energy in the signal from the low pass filter ( 25 ), at which an alarm condition would be determined as having been detected, is increased to an intermediate threshold value ( 40 ′″) which is above the 25 energy value of the low frequency component of the mobile phone interference, thereby preventing a false alarm due to the presence of mobile phone interference. However, the intermediate value ( 40 ′″) of the low frequency energy alarm threshold is such as to permit detection for slow movements in the vehicle during the presence of the mobile phone interference.

The present invention relates to a method and apparatus for analysing afrequency signal from a movement detector for determining if movementhas been detected in an area under surveillance by the movementdetector, and in particular, the invention relates to such a method andapparatus for analysing the frequency signal for distinguishing a signalresulting from movement from a signal resulting from a spurious source.The invention also relates to an anti-theft system for detectingmovement in an area under surveillance, for example, for detectingmovement in a motor vehicle.

Movement detectors for detecting movement in an area under surveillanceare well known. Such movement detectors are commonly used in anti-theftsystems for motor vehicles. In general, such movement detectors operateon the Doppler principle, and comprise a radio frequency transmitter,which transmits a radio frequency signal which is detected by anadjacent receiver, which also receives a reflection of the transmittedsignal. The received transmitted signal and the received reflectedtransmitted signal are mixed to produce a difference frequency signalwhich is equal to the Doppler shift produced by a moving target fromwhich the transmitted signal is reflected. In order to precisely definethe range of such movement detectors, the radio frequency signal istransmitted in short bursts, and the length of the burst determines thesurveillance range. Such a movement detector is disclosed in U.S. Pat.No. 5,966,090 of Thomas E. McEwan. While movement detectors of the typeof the McEwan detector are suitable for detecting movement in a motorvehicle, for example, movement caused by an attempt to gain entry to orto steal the vehicle, unfortunately, such detectors are, in general,unsuitable for distinguishing between a movement resulting from anattempt to enter the vehicle or to steal the vehicle and externalinterference, such as, for example, the type caused by a mobile phoneand other such interference. Thus, such movement detectors are prone togiving false alarms.

Where such movement detectors are capable of determining that a signalreceived is not the result of movement, such detectors classify suchsignals as being the result of interference, and while the interferenceremains the detector is inhibited from outputting an alarm signal, inorder to avoid a potential false alarm. This, thus, causes seriousproblems in that a determined thief could, for example, by generating amobile phone signal in or in proximity to a motor vehicle, inhibit amovement detector of such a vehicle from outputting an alarm signal, andcould thus gain entry to the vehicle without the alarm being activated.A further problem associated with such movement detectors is that it hasbeen found that where coins or other light metal objects are placedtogether in a tray in a motor vehicle, for example, in an ashtray or thelike, and if the vehicle is subjected to a jolt the coins commence tovibrate. The vibrating coins induce a high energy interference signalwhich is received by the detector. This is thus another source of falsealarms, and to avoid such a false alarm again in the presence of such asignal, the movement detector is inhibited from outputting an alarmsignal. Vibration of such coins or other light metal objects within avehicle can be caused by a person banging on the vehicle, or indeed, byhailstones or particularly heavy rain falling on the vehicle. Thus, athief by banging on the vehicle could induce vibration in coins in avehicle sufficient to disable the movement detector, thus permitting thethief to gain entry to the vehicle without the alarm being activated.This type of interference caused by vibrating coins in a vehicle iscommonly referred to as passive intermodulation interference.

There is therefore a need for a method and apparatus for analysingsignals from a movement detector which permits a signal resulting frommovement to be distinguished from a signal resulting from a spurioussignal, for minimising periods during which an anti-theft system isdisabled in the presence of a spurious signal, while at the same timeminimising false alarms. There is also a need for an anti-theft systemfor detecting movement to be distinguished from signals from a spurioussource.

The present invention is directed towards providing such a method andapparatus for analysing a frequency signal from a movement detector fordistinguishing a signal resulting from movement from a signal resultingfrom a spurious signal, and the invention is also directed to ananti-theft system.

According to the invention there is provided a method for analysing afrequency signal from a movement detector for determining if movementhas been detected in an area under surveillance by the movementdetector, and for distinguishing a signal resulting from movement from asignal resulting from a spurious source, wherein the method comprisesthe steps of:

-   -   determining a value of a characteristic of a low frequency        component of the frequency signal,    -   comparing the determined value of the characteristic of the low        frequency component with a set threshold value of a low        frequency alarm threshold for determining if the determined        value of the characteristic of the low frequency component is        indicative of movement having been detected,    -   determining a value of a characteristic of a high frequency        component of the frequency signal to ascertain if the value of        the characteristic of the high frequency component is indicative        of a signal from a known spurious source having been detected,        and    -   setting the low frequency alarm threshold at an intermediate        threshold value, if the determined value of the characteristic        of the high frequency component is indicative of a signal from a        known spurious source, the intermediate threshold value being        greater than the value of a corresponding characteristic of a        low frequency component of the frequency signal resulting from        the signal from the known spurious source for preventing a false        alarm.

In one embodiment of the invention the method further comprisesoutputting an alarm signal in response to the determined value of thecharacteristic of the low frequency component of the frequency signalexceeding the low frequency alarm threshold when the low frequency alarmthreshold is set at the intermediate threshold value.

In another embodiment of the invention an alarm signal is outputted inresponse to the determined value of the characteristic of the lowfrequency component of the frequency signal exceeding the low frequencyalarm threshold when the low frequency alarm threshold is set at a firstthreshold value, and when the determined characteristic of the highfrequency component of the frequency signal does not exceed a highfrequency threshold, the first threshold value of the low frequencyalarm threshold being less than the intermediate threshold valuethereof.

In a further embodiment of the invention the determined value of thecharacteristic of the high frequency component of the frequency signalis compared with the high frequency threshold for determining if thevalue of the characteristic of the high frequency component isindicative of a signal from a spurious source, and if the determinedvalue of the characteristic of the high frequency component exceeds thehigh frequency threshold, the low frequency alarm threshold is set at amaximum threshold value.

Preferably, the maximum threshold value of the low frequency alarmthreshold is greater than the intermediate threshold value thereof forpreventing false alarms.

Advantageously, the maximum threshold value of the low frequency alarmthreshold is greater than the greatest value of the characteristic ofthe low frequency component of frequency signals likely to beencountered.

Preferably, the first threshold value of the low frequency alarmthreshold is set at a value above the background noise level in the lowfrequency component of the frequency signal in the absence of a spurioussignal. Advantageously, the first threshold value of the low frequencyalarm threshold is set in response to the value of the background noisein the low frequency component of the frequency signal.

In one embodiment of the invention the intermediate threshold value ofthe low frequency alarm threshold is set in response to the determinedvalue of the characteristic of the high frequency component of thefrequency signal resulting from the signal from the known spurioussource.

Preferably, the determined value of the characteristic of the highfrequency component of the frequency signal is compared with thecorresponding determined characteristic of the low frequency componentof the frequency signal for determining if the frequency signal hasresulted from a signal from a known spurious source.

Advantageously, the ratio of the determined value of the characteristicof the high frequency component of the frequency signal to thedetermined value of the characteristic of the low frequency component ofthe frequency signal is compared with a predetermined ratio value, fordetermining if the frequency signal resulted from a known spurioussource.

In one embodiment of the invention the frequency range of the lowfrequency component of the frequency signal is selected for determiningslow movements within the area under surveillance.

In another embodiment of the invention the method further comprises thestep of determining the value of a characteristic of an intermediatefrequency component of the frequency signal, and comparing thedetermined value of the characteristic of the intermediate frequencycomponent with an intermediate frequency threshold for determining ifthe determined value of the characteristic of the intermediate frequencycomponent of the frequency signal is indicative of a fast movementhaving been detected.

In another embodiment of the invention the intermediate frequency rangeof the frequency signal is in the range of 40 Hz to 100 Hz. Preferably,the intermediate frequency range of the frequency signal is in the rangeof 40 Hz to 75 Hz.

In another embodiment of the invention the low frequency range of thefrequency signal is in the range up to 100 Hz. Preferably, the lowfrequency range of the frequency signal is in the range up to 75 Hz.Advantageously, the low frequency range of the frequency signal is inthe range up to 40 Hz.

In a further embodiment of the invention the high frequency range of thefrequency signal exceeds 100 Hz. Preferably, the high frequency range ofthe frequency signal is in the range of 100 Hz to 1,000 Hz.

In one embodiment of the invention the set threshold value of the lowfrequency alarm threshold is reduced when a signal from a spurioussource is determined as having ceased. Preferably, the set thresholdvalue of the low frequency alarm threshold is progressively reduced whena signal from a spurious source is determined as having ceased.Advantageously, the set-threshold value of the low frequency alarmthreshold is progressively reduced in decremental steps when a signalfrom a spurious source is determined as having ceased. Ideally, the setthreshold value of the low frequency alarm threshold is reduced to thefirst threshold value when the signal from a spurious source isdetermined as having ceased.

In one embodiment of the invention the characteristic, the value ofwhich is determined in respect of the low frequency component of thefrequency signal is a characteristic indicative of the energy of the lowfrequency component of the frequency signal, and the low frequency alarmthreshold is an energy threshold.

In another embodiment of the invention the characteristic, the value ofwhich is determined in respect of the low frequency component of thefrequency signal is a characteristic indicative of the amplitude of thelow frequency component of the frequency signal, and the low frequencyalarm threshold is an amplitude threshold.

Preferably, the value of the characteristic indicative of the energy andthe value of the characteristic indicative of the amplitude of the lowfrequency component of the frequency signal are both determined.

In one embodiment of the invention the characteristic, the value ofwhich is determined in respect of the high frequency component of thefrequency signal is a characteristic indicative of the energy of thehigh frequency component of the frequency signal, and the high frequencythreshold is an energy threshold.

In another embodiment of the invention the characteristic, the value ofwhich is determined in respect of the intermediate frequency componentof the frequency signal is a characteristic indicative of the energy ofthe intermediate frequency component of the frequency signal, and theintermediate frequency threshold is an energy threshold.

In a further embodiment of the invention the characteristic, the valueof which is determined in respect of the intermediate frequencycomponent of the frequency signal is a characteristic indicative of theamplitude of the intermediate frequency component of the frequencysignal, and the intermediate frequency threshold is an amplitudethreshold.

Preferably, the value of the characteristic indicative of the energy andthe value of the characteristic indicative of the amplitude of theintermediate frequency component of the frequency signal are bothdetermined.

In one embodiment of the invention the value of the characteristicindicative of energy of each frequency component of the frequency signalis determined by integrating the corresponding frequency component ofthe frequency signal.

In another embodiment of the invention the low, high and intermediatefrequency components of the frequency signal are sampled atpredetermined intervals during a predetermined cycle period.

In a further embodiment of the invention the maximum value of thecharacteristic determined during each cycle period for each frequencycomponent is recorded, and the alarm signal is outputted in response tothe recorded maximum values.

In a still further embodiment of the invention the number of times thedetermined value of the characteristic indicative of the amplitude ofthe low frequency component of the frequency signal exceeds the setamplitude threshold value of the low frequency alarm threshold isrecorded during each cycle period, and the alarm signal is outputted inresponse to the recorded number of times the determined value of thecharacteristic indicative of the amplitude exceeds the set amplitudethreshold value of the low frequency alarm threshold.

In one embodiment of the invention the number of times the determinedvalue of the characteristic indicative of the amplitude of theintermediate frequency component of the frequency signal exceeds the setamplitude threshold value of the intermediate frequency threshold isrecorded during each cycle period, and the alarm signal is outputted inresponse to the recorded number of times the determined value of thecharacteristic indicative of the amplitude exceeds the set amplitudethreshold value of the intermediate frequency threshold.

In another embodiment of the invention the frequency signal is passedthrough a low pass filter for providing the low frequency component ofthe frequency signal. Preferably, the frequency signal is passed througha high pass filter for providing the high frequency component of thefrequency signal. Advantageously, the frequency signal is passed througha bandpass filter for providing the intermediate frequency component ofthe frequency signal.

In one embodiment of the invention the frequency signal is derived froma microwave movement detector.

In another embodiment of the invention the frequency signal is derivedfrom a differential microwave signal.

In a further embodiment of the invention the frequency signal is derivedfrom an envelope detector.

In a still further embodiment of the invention the frequency signal isderived from a receiver which receives a combination of a transmittedsignal and a reflection of the transmitted signal.

Preferably, the transmitted signal is transmitted in bursts.

Advantageously, the received transmitted signal and the reflection ofthe transmitted signal are mixed to produce a difference frequencycorresponding to the Doppler shift produced by a moving target.

In one embodiment of the invention the frequency signals are voltagesignals.

Additionally, the invention provides apparatus for analysing a frequencysignal from a movement detector for determining if movement has beendetected in an area under surveillance by the movement detector, and fordistinguishing a signal resulting from movement from a signal resultingfrom a spurious source, wherein the apparatus comprises:

-   -   a means for determining a value of a characteristic of a low        frequency component of the frequency signal,    -   a first comparing means for comparing the determined value of        the characteristic of the low frequency component with a set        threshold value of a low frequency alarm threshold for        determining if the determined value of the characteristic of the        low frequency component is indicative of movement having been        detected,    -   a means for determining a value of a characteristic of a high        frequency component of the frequency signal to ascertain if the        value of the characteristic of the high frequency component is        indicative of a signal from a known spurious source having been        detected, and    -   a means for setting the low frequency alarm threshold at an        intermediate threshold value, if the determined value of the        characteristic of the high frequency component is indicative of        a signal from a known spurious source, the intermediate        threshold value being greater than the value of a corresponding        characteristic of a low frequency component of the frequency        signal resulting from the signal from the known spurious source        for preventing a false alarm.

In one embodiment of the invention a means is provided for outputting analarm signal, the means for outputting the alarm signal being responsiveto the determined value of the characteristic of the low frequencycomponent of the frequency signal exceeding the low frequency alarmthreshold when the low frequency alarm threshold is set at theintermediate threshold value.

In another embodiment of the invention the means for outputting thealarm signal is responsive to the determined value of the characteristicof the low frequency component of the frequency signal exceeding the lowfrequency alarm threshold when the low frequency alarm threshold is setat a first threshold value, and when the determined characteristic ofthe high frequency component of the frequency signal does not exceed ahigh frequency threshold, the first threshold value of the low frequencyalarm threshold being less than the intermediate threshold valuethereof.

In a further embodiment of the invention a second comparing means isprovided for comparing the determined value of the characteristic of thehigh frequency component of the frequency signal with the high frequencythreshold for determining if the value of the characteristic of the highfrequency component is indicative of a signal from a spurious source,and if the determined value of the characteristic of the high frequencycomponent exceeds the high frequency threshold, the means for settingthe threshold value of the low frequency alarm threshold sets the lowfrequency alarm threshold at a maximum threshold value. Preferably, themaximum threshold value of the low frequency alarm threshold is greaterthan the intermediate threshold value thereof for preventing falsealarms. Advantageously, the maximum threshold value of the low frequencyalarm threshold is greater than the greatest value of the characteristicof the low frequency component of frequency signals likely to beencountered.

In one embodiment of the invention the means for setting the thresholdvalue of the low frequency alarm threshold sets the low frequency alarmthreshold at the first threshold value, which is a value above thebackground noise level in the low frequency component of the frequencysignal, in the absence of spurious signals.

Preferably, the means for setting the low frequency alarm threshold isresponsive to the value of the background noise in the low frequencycomponent of the frequency signal for setting the low frequency alarmthreshold at the first threshold value.

Advantageously, the means for setting the low frequency alarm thresholdsets the low frequency alarm threshold at the intermediate thresholdvalue in response to the determined value of the characteristic of thehigh frequency component of the frequency signal resulting from thesignal from the known spurious source.

In another embodiment of the invention a third comparing means isprovided for comparing the determined value of the characteristic of thehigh frequency component of the frequency signal with the correspondingdetermined characteristic of the low frequency component of thefrequency signal for determining if the frequency signal has resultedfrom a signal from a known spurious source.

Preferably, the third comparing means compares the ratio of thedetermined value of the characteristic of the high frequency componentof the frequency signal to the determined value of the characteristic ofthe low frequency component of the frequency signal with a predeterminedratio value for determining if the frequency signal has resulted from asignal from a known spurious source.

Preferably, the frequency range of the low frequency component of thefrequency signal is selected for determining slow movements within thearea under surveillance.

In one embodiment of the invention a means for determining the value ofa characteristic of an intermediate frequency component of the frequencysignal is provided, and a fourth comparing means is provided forcomparing the determined value of the characteristic of the intermediatefrequency component with an intermediate frequency threshold fordetermining if the determined value of the characteristic of theintermediate frequency component of the frequency signal is indicativeof fast movement having been detected.

In another embodiment of the invention the intermediate frequency rangeof the frequency signal is in the range of 40 Hz to 100 Hz. Preferably,the intermediate frequency range of the frequency signal is in the rangeof 40 Hz to 75 Hz.

In another embodiment of the invention the low frequency range of thefrequency signal is in the range up to 100 Hz. Preferably, the lowfrequency range of the frequency signal is in the range up to 75 Hz.Advantageously, the low frequency range of the frequency signal is inthe range up to 40 Hz.

In another embodiment of the invention the high frequency range of thefrequency signal exceeds 100 Hz. Preferably, the high frequency range ofthe frequency signal is in the range of 100 Hz to 1,000 Hz.

In one embodiment of the invention the means for setting the thresholdvalue of the low frequency alarm threshold reduces the low frequencyalarm threshold in response to a signal from a spurious source havingceased. Preferably, the means for setting the threshold value of the lowfrequency alarm threshold progressively reduces the low frequency alarmthreshold in response to a spurious source having ceased.Advantageously, the means for setting the threshold value of the lowfrequency alarm threshold progressively reduces the low frequency alarmthreshold in decremental steps in response to a spurious source havingceased. Ideally, the means for setting the threshold value of the lowfrequency alarm threshold reduces the low frequency alarm threshold tothe first threshold value in response to a spurious source havingceased.

In another embodiment of the invention the means for determining thevalue of the characteristic which is determined in respect of the lowfrequency component of the frequency signal determines the value of acharacteristic indicative of the energy of the low frequency componentof the frequency signal, and the low frequency alarm threshold is anenergy threshold.

In another embodiment of the invention the means for determining thevalue of the characteristic which is determined in respect of the lowfrequency component of the frequency signal determines the value of acharacteristic indicative of the amplitude of the low frequencycomponent of the frequency signal, and the low frequency alarm thresholdis an amplitude threshold.

Preferably, the means for determining the value of the characteristic ofthe low frequency component of the frequency signal determines thevalues of the characteristic indicative of the energy and thecharacteristic indicative of the amplitude of the low frequencycomponent of the frequency signal.

In another embodiment of the invention the means for determining thevalue of the characteristic which is determined in respect of the highfrequency component of the frequency signal determines the value of acharacteristic indicative of the energy of the high frequency componentof the frequency signal, and the high frequency threshold is an energythreshold.

In one embodiment of the invention the means for determining the valueof the characteristic which is determined in respect of the intermediatefrequency component of the frequency signal determines the value of acharacteristic indicative of the energy of the intermediate frequencycomponent of the frequency signal, and the intermediate frequencythreshold is an energy threshold.

In another embodiment of the invention the means for determining thevalue of the characteristic which is determined in respect of theintermediate frequency component of the frequency signal determines acharacteristic indicative of the amplitude of the intermediate frequencycomponent of the frequency signal, and the intermediate frequencythreshold is an amplitude threshold.

Preferably, the means for determining the value of the characteristic ofthe intermediate frequency component of the frequency signal determinesthe values of the characteristic indicative of the energy and thecharacteristic indicative of the amplitude of the intermediate frequencycomponent of the frequency signal.

In one embodiment of the invention the characteristic indicative ofenergy of each frequency component of the frequency signal is determinedby an integrating means which integrates the corresponding frequencycomponent of the frequency signal.

In another embodiment of the invention a sampling means is provided forsampling the low, high and intermediate frequency components of thefrequency signal at predetermined intervals during a predetermined cycleperiod.

In a further embodiment of the invention a storing means is provided forstoring the maximum value of the characteristic determined during eachcycle period for each frequency component, and the means for outputtingthe alarm signal is responsive to the stored maximum values. Preferably,the storing means stores the number of times the determined value of thecharacteristic indicative of the amplitude of the low frequencycomponent of the frequency signal exceeds the set threshold value of thelow frequency alarm threshold during each cycle period, and the meansfor outputting the alarm signal is responsive to the stored number oftimes the determined value of the characteristic indicative of theamplitude exceeds the set threshold value of the low frequency alarmthreshold.

Advantageously, the storing means stores the number of times thedetermined value of the characteristic indicative of the amplitude ofthe intermediate frequency component of the frequency signal exceeds theintermediate frequency threshold during each cycle period, and the meansfor outputting the alarm signal is responsive to the recorded number oftimes the determined value of the characteristic indicative of theamplitude exceeds the intermediate frequency threshold.

In one embodiment of the invention a low pass filter is provided forfiltering the frequency signal for producing the low frequency componentof the frequency signal. Preferably, a high pass filter is provided forfiltering the frequency signal for producing the high frequencycomponent of the frequency signal. Advantageously, a bandpass filter isprovided for filtering the frequency signal for producing theintermediate frequency component of the frequency signal.

In one embodiment of the invention a microwave movement detector isprovided for providing the frequency signal.

In another embodiment of the invention the frequency signal is derivedfrom a differential microwave signal.

Preferably, the microwave movement detector comprises an envelopedetector for outputting the frequency signal.

Advantageously, the microwave detector comprises a transmitter circuitfor transmitting a radio frequency signal and a receiver circuit forreceiving a combination of the transmitted signal and a reflection ofthe transmitted signal.

Preferably, the transmitter circuit transmits the radio frequency signalin bursts.

Advantageously, the receiver circuit mixes the received transmittedsignal and the reflection of the transmitted signal to produce adifference frequency corresponding to the Doppler shift produced by amoving target.

In one embodiment of the invention the frequency signals are voltagesignals.

The invention also provides an anti-theft system comprising atransmitter circuit for transmitting a sequence of bursts ofelectromagnetic energy to produce a sensor field, the transmitted burstshaving burst widths which vary according to a pattern, a receivercircuit for receiving a combination of the transmitted bursts andreflections of the transmitted bursts and for producing a combinedoutput frequency signal, and apparatus according to the invention forreceiving and analysing the frequency signal.

The advantages of the invention are many. By virtue of the fact that themethod and apparatus according to the invention can identify a signalfrom a known spurious source, and furthermore, by virtue of the factthat on identifying a signal from the known spurious source the lowfrequency alarm threshold is set at an intermediate threshold value,which is greater than the value of the corresponding characteristic ofthe low frequency component of the frequency signal resulting from thesignal from the known spurious source, false alarms are avoided, andfurthermore, the anti-theft system can still detect movement in thepresence of the signal from the known spurious source, by determiningthe value of the characteristic of the low frequency component of thefrequency signal. Thus, if the determined value of the characteristic ofthe low frequency component of the frequency signal exceeds theintermediate threshold value, a valid alarm signal is outputted. This isa particularly important advantage in that it permits the anti-theftsystem to continue to operate in the presence of the signal from theknown spurious source, and detect movement in the presence of the knownspurious source signal. Where the signal from the known spurious sourceis a mobile phone signal, the anti-theft system can still operate in thepresence of a mobile phone signal. This is a particularly importantadvantage where the anti-theft system is installed in a motor vehiclefor detecting movement in the motor vehicle.

The invention will be more clearly understood from the followingdescription of a preferred embodiment thereof, which is given by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram of an anti-theft system according to theinvention,

FIG. 2 is a graphical representation of a number of waveforms of signalsprocessed by the anti-theft system of FIG. 1,

FIG. 3 illustrates a flow chart of a software routine under which amicroprocessor of the anti-theft system of FIG. 1 operates,

FIG. 4 illustrates a flow chart of a software subroutine of the routineof FIG. 3, and

FIG. 5 illustrates a flow chart of another subroutine of the softwareroutine of FIG. 3.

Referring to the drawings and initially to FIG. 1, there is illustratedan anti-theft system according to the invention indicated generally bythe reference numeral 1 for monitoring for movement in an area undersurveillance, in this case in a motor vehicle, resulting from a break-inor an attempted break-in to the vehicle. The anti-theft system 1comprises a low power microwave differential pulse Doppler movementdetector indicated generally by the reference numeral 2, for detectingthe movement in the vehicle. The anti-theft system 1 can distinguishbetween movement resulting from an attempted break-in or a break-in intothe vehicle and a signal from a known spurious source, for example,interference from a known spurious source, which in this embodiment ofthe invention is a spurious signal caused by interference from a mobilephone signal in the motor vehicle, and as will be described below, theanti-theft system 1 can operate in the presence of mobile phoneinterference, and still detect movement in the vehicle resulting from anattempt to break into the vehicle or movement of a thief within thevehicle, and distinguish the movement from the mobile phoneinterference, and output a valid alarm signal in response to thedetected movement.

The detector 2 is of the type which is disclosed in U.S. Pat. No.5,966,090 of Thomas E. McEwan, and the disclosure in the U.S.specification is incorporated herein by reference. Briefly, the detector2 comprises a transmitter circuit 3 and a receiver circuit 4. Thetransmitter circuit 3 transmits bursts of electromagnetic energy, inthis case radio frequency signal bursts of two burst widths at afrequency typically of approximately 5.8 GHz. The burst widths definethe maximum sensing distance of the detector 2, and in this embodimentof the invention the burst widths of the respective bursts are typicallyapproximately 5.5 nanoseconds and 6 nanoseconds for limiting the sensingdistance to 1 meter approximately. The receiver circuit 4 receives thetransmitted radio frequency signal bursts and reflected signal burstsfrom a target in the vehicle, and the reflected signal bursts are mixedto produce a difference frequency equal to the Doppler shift produced bya moving target within the vehicle, for example, movement of a hand orarm of a thief within the vehicle.

The transmitter circuit 3 comprises a pulse repetition frequencyoscillator 5 which provides a square wave pulse train at a pulserepetition rate of approximately 1.5 MHz. A noise generating element 6modulates the output signal of the oscillator 5 in order to spreadradiated spectral lines that would otherwise occur with uniform spacingequal to pulse repetition frequency. A gate generator 7 is driven by theoscillator 5 to produce the frequency bursts of approximately 6nanoseconds under the control of a burst width control circuit 9. Theburst width control circuit 9 comprises a range oscillator 10, and arange setting potentiometer 11. A summing node 12 combines the output ofthe range oscillator 10 and the potentiometer 11 for producing a pulsewidth control signal to the gate generator 7. The operation of a controlcircuit similar to the control circuit 9 is described in U.S. Pat. No.5,966,090 of McEwan, and it is not intended to describe it in furtherdetail here. The output of the gate generator 12 activates a radiofrequency oscillator 14 for the duration of the pulse output from thegate generator 7, which generates the radio frequency signal at afrequency of approximately 5.8 GHz in the respective burst widths. Aloop antenna transmitter 15 transmits the radio frequency signal burstsfrom the radio frequency generator 14, which are received on anadjacently located loop antenna receiver 17, which also receives thereflected transmitted signals reflected from the target.

Turning now to the receiver circuit 3, the sum of the transmitted radiofrequency signal and the reflected transmitted radio frequency signalsare fed from the loop antenna receiver 17 to a radio frequency detector18. The radio frequency detector 18 detects the peak radio frequencyenvelope, and substantially holds the peak value from one burst to thenext. However, the drop rate is such that it can follow the patternfrequency burst with variations established by the range oscillator 10.An intermediate frequency filter 19 is tuned to pass the burst widthvariation rate and to reject DC offsets from the radio frequencydetector 18 and unmodulated Doppler variation. An intermediate frequencyamplifier 20 amplifies the filtered signal from the intermediatefrequency filter 19. The output of the intermediate frequency amplifier20 is fed to an envelope detector 22 where one of the high or low levelsof the signal from the amplifier 20 is peak detected or envelopedetected to provide a resulting differential signal. The output of theenvelope detector 22 corresponds to the difference in the response ofthe radio frequency detector 18 between the two radio frequency burstwidths, or between the two sensor ranges established by the varyingburst widths.

The envelope signal outputted by the envelope detector 22 is fed throughthree analogue filters implemented in hardware, namely, a low passfilter 25, a high pass filter 26 and a bandpass filter 27. The frequencysignals outputted by the three filters 25, 26 and 27 are fed toapparatus indicated generally by the reference numeral 34 for analysisthereof for determining if the frequency signal outputted by theenvelope detector 22 is indicative of movement having been detected, orif it is indicative of interference, and if the interference is mobilephone interference. The apparatus 34 and its operation is describedbelow. The low pass filter 25 passes frequency signals of frequencypredominantly in the range of 1 Hz to 40 Hz. The high pass filter passesfrequency signals of frequency predominantly in the range of 100 Hz to1,000 Hz, while the bandpass filter passes frequency signals offrequency predominantly in the range of 40 Hz to 80 Hz approximately. Ingeneral, it has been found that slow movements, for example an arm orhand of a person moving at a normal relatively slow rate are detected inthe low frequency component of the signal from the envelope detector 22,in other words, in signals in the frequency range 1 to 40 Hz from thelow pass filter 25. Fast movements of a hand or arm of an individualtypically are detected in an intermediate frequency component of thesignal from the envelope detector 22, in other words, in signals in theintermediate frequency range of 40 Hz to 80 Hz from the bandpass filter27. Interference signals from spurious sources as well as being detectedin the low and intermediate frequency ranges are also detected in thehigh frequency component of the signal from the envelope detector 22, inother words, in signals from the high pass filter 26. Interferencesignals resulting from passive intermodulation which is caused byvibrating coins or vibrating light metal objects within a vehicle arefound at all frequencies from low frequencies through intermediatefrequencies to high frequencies above 100 Hz. Similarly, interferencesignals caused by mobile phones are also found at all frequencies fromlow frequencies through intermediate frequencies to high frequenciesabove 100 Hz. However, it has been found that there is an identifiablerelationship between the high frequency component and the low frequencycomponent of the frequency signals outputted by the high pass and lowpass filters 26 and 25, respectively resulting from mobile phoneinterference. It has been found that the ratio of the energy of the highfrequency component of mobile phone interference to the energy of thelow frequency component in general exceeds a predetermined ratio value.The predetermined ratio value is dependent on the method for computingthe energies in the respective high frequency and low frequencycomponents of the interference. This discovery has been made use of inthe present invention in order to firstly, identify the presence ofinterference from a mobile phone, and secondly, to adjust thesensitivity of the anti-theft system 1 in order that the anti-theftsystem 1 can continue to operate in the presence of interference causedby a mobile phone, and can continue to monitor for movement in thepresence of a mobile phone.

However, in the case of passive intermodulation interference the energylevel of the interference signal in the three frequency ranges is suchas to prevent differentiation between such passive intermodulationinterference and a signal resulting from a slow or fast movement of anindividual in a vehicle. Thus, the anti-theft system 1 is operatedduring the presence of interference caused by passive intermodulationnot to output an alarm signal in order to avoid false alarms.

The apparatus 34 comprises a microprocessor 28, and the filtered signalsfrom the filters 25, 26 and 27 are fed to the microprocessor 28 wherethey are processed and analysed as will be described below. An output 29from the microprocessor 28 outputs an alarm signal to an alarm circuit35 for activating an alarm in the event of movement being detected inthe motor vehicle. The alarm circuit 35 may activate an audible and/orvisual alarm, and may also activate a transmitter for transmitting amessage to a central monitoring station indicating that an alarmcondition exists in the vehicle.

The microprocessor 28 under the control of software which will bedescribed below determines values of characteristics of the low,intermediate and high frequency components from the respective filters25, 26 and 27 of the frequency signal from the envelope detector 22. Inthis embodiment of the invention one of the characteristics, the valuesof which are determined, is a characteristic indicative of the energiesof the low, intermediate and high frequency components of the signal,and the other characteristic the values of which are determined is acharacteristic indicative of the amplitudes of the low and intermediatefrequency components of the signal. Before describing the operation ofthe microprocessor 28, the signals outputted by the filters 25, 26 and27, as well as the energy levels of the signals determined by themicroprocessor 28 will first be described with reference to FIG. 2.

The frequency signals outputted by the low pass filter 25, the high passfilter 26 and the bandpass filter 27 are voltage signals, and arecontinuously sampled by the microprocessor 28 at 2.04 millisecondintervals. The microprocessor 28 operates on a cycle period of 500milliseconds, and on the end of each 500 millisecond cycle period, thenext cycle period commences. Thus, the signals outputted by the threefilters 25, 26 and 27 are each sampled two hundred and forty-four timesduring each 500 millisecond cycle period. Waveform A of FIG. 2 is atrace of the voltage sampled by the microprocessor 28 of the lowfrequency component of the frequency signal outputted by the low passfilter 25. The voltage of the intermediate frequency component of thefrequency signal outputted by the bandpass filter 27 sampled by themicroprocessor 28 is illustrated by waveform B of FIG. 2. Waveform C ofFIG. 2 illustrates the sampled voltage of the high frequency componentof the signal outputted by the high pass filter 26. Waveform D of FIG. 2represents the energy of the low frequency component of the signaloutputted by the low pass filter 25, which is derived by themicroprocessor 28 from the waveform A. Waveform E of FIG. 2 representsthe energy in the intermediate frequency component of the signaloutputted by the bandpass filter 27, which is derived by themicroprocessor 28 from the waveform B. Waveform F of FIG. 2 representsthe energy in the high frequency component of the signal outputted bythe high pass filter 26, which is derived by the microprocessor 28 fromthe waveform C. The energy in the respective low frequency, intermediatefrequency and high frequency components of the signal are determined inarbitrary units by algorithms.

The algorithm for determining the value of the energy of the lowfrequency component of the signal is as follows. A subroutine of a mainsoftware routine which is described below with reference to FIG. 3continuously samples the low frequency component of the frequency signalreceived from the low pass filter 25 at a rate of 488 samples persecond, and the DC mean of the low frequency component of the signal isfound. The voltage difference between each sample and the DC mean isthen calculated, and these values are passed through a low pass filterimplemented in software in the microprocessor 28 which operates in thefrequency range 1 Hz to 40 Hz. The output of the software low passfilter is indicative of the energy present in the low frequencycomponent of the signal outputted by the low pass filter 25. A similarsubroutine determines the energy present in the intermediate frequencycomponent of the frequency signal outputted by the bandpass filter 27. Acorresponding software bandpass filter is implemented in software in themicroprocessor 28, which operates in the frequency range 40 Hz to 80 Hz,and the output of the software bandpass filter is indicative of theenergy present in the intermediate frequency component of the frequencysignal outputted by the bandpass filter 27. The energy present in thehigh frequency component of the frequency signal outputted by the highpass filter 26 is determined by a subroutine of the main routine. Thehigh frequency component of the frequency signal outputted by the highpass filter 27 is continuously sampled at a rate of 488 samples persecond, and the sampled values are passed through a high pass filterimplemented in software in the microprocessor 28. The software high passfilter in the microprocessor 28 has a cut-off frequency of 150 Hz. Theoutput of the software high pass filter is indicative of the energypresent in the high frequency component of the frequency signaloutputted by the high pass filter 26.

Since movements of the type which it is required to detect appear in thelow frequency component of the signal from the low pass filter 25 andthe intermediate frequency component from the bandpass filter 27, upperand lower low frequency voltage amplitude thresholds 30 and 31,respectively, are set for the voltage of the waveform A, and upper andlower intermediate frequency voltage amplitude thresholds 32 and 33,respectively, are set for the voltage of the waveform B. The upper andlower amplitude thresholds 30, 31, 32 and 33 are set and maintained bythe microprocessor 28 to be about ten percent above and below thebackground noise in the respective low and intermediate frequencycomponents of the signal. Each sampled voltage from the low pass andbandpass filters 25 and 27 is compared with the corresponding upper andlower thresholds 30, 32, 33 and 34 to obtain an indication if a movementhas been detected, or if interference is present. The low, intermediateand high frequency components of the signal are illustrated from time t₀to t₁ in FIG. 2 with background noise only.

Additionally, a low frequency energy alarm threshold 40 is set for theenergy of the low frequency component of the signal, namely, thewaveform D, at which an alarm condition may be determined as havingoccurred. The low frequency energy alarm threshold is varied as will bedescribed below in order to avoid false alarms. An intermediatefrequency energy threshold 42 is set for the energy of the intermediatefrequency component of the signal, namely, waveform E at which an alarmcondition may also be determined as having occurred. The intermediatefrequency energy threshold is set and maintained at about ten percentabove the average energy of the background noise in the low frequencycomponent of the frequency signal. A high frequency energy threshold 44is set for the energy of the high frequency component of the signal,namely, the waveform F, at which interference is determined as beingpresent. The high frequency energy threshold 44 is set and maintained atabout ten percent above the average energy of the background noise inthe high frequency component of the frequency signal.

Referring in particular to waveform A of FIG. 2 which is the sampledvoltage waveform of the low frequency component of the signal from thelow pass filter 25. Initially from time t₀ up to time t₁ only backgroundnoise is detected in the low frequency component of the signal, and thesignal is well within the upper and lower voltage thresholds 30 and 31.Similarly, the voltage waveforms B and C of the intermediate and highfrequency components of the signal sampled from the bandpass filter 27and the high pass filter 26 from time t₀ to time t₁ show only backgroundnoise being present, and the waveform B is well within the upper andlower voltage thresholds 32 and 33. Because of the fact that onlybackground noise is present in the detected signal, the low frequencyenergy alarm threshold 40 is set and maintained at a first thresholdvalue 40′ which is approximately ten percent above the average energy ofthe background noise in the low frequency component of the signal, forso long as only background noise is detected.

From time t₁ to time t₂ a disturbance is detected in the voltage signalof the low frequency component of the signal, waveform A, and theamplitude of the voltage signal is exceeding the upper and loweramplitude thresholds 30 and 31. However, between time t₁ and time t₂only a small disturbance is detected in the voltage waveforms B and C ofthe intermediate and high frequency components of the signals, and theamplitude of waveform B is within the upper and lower amplitudethresholds 32 and 33. The energy of the low frequency component of thesignal, namely, waveform D just after time t₁ exceeds the low frequencyenergy alarm threshold, which is set at the first threshold value 40′for the duration of time t₁ to time t₂. The energy in the high frequencycomponent of the signal, waveform F, has not exceeded the high frequencyenergy threshold 44, thus indicating the absence of interference, forexample, interference from a mobile phone signal or passiveintermodulation. Therefore, since the energy of the low frequencycomponent has exceeded the set low frequency energy alarm threshold 40′and no interference is present, and since the amplitude of the lowfrequency component of the signal is exceeding the upper and loweramplitude thresholds 30 and 31, a movement has been detected, and analarm condition exists. At this stage as will be described below themicroprocessor 28 outputs an alarm signal on the output 29 to the alarmcircuit 35.

From time t₂ to time t₃, only background noise is detected in the low,intermediate and high frequency components of the signal, and the lowfrequency energy alarm thresholds 40 are left unaltered at the firstthreshold value of 40′.

At time t₃ a disturbance is detected in the voltage waveforms A, B and Cof the low, intermediate and high frequency components of the signal.Since the disturbance is detected in the high frequency component of thesignal, and the energy of the high frequency component, waveform Fexceeds the high frequency energy threshold 44, interference is present,and in order to avoid a false alarm, the low frequency energy alarmthreshold 40 is increased to a maximum threshold value 40″. The maximumthreshold value of the low frequency energy alarm threshold is of valuesuch that it would not be exceeded by the energy of any low frequencycomponent signal detected in order to avoid false alarms. During thepresent of any interference, in other words when the energy in the highfrequency component of the signal exceeds the high frequency energythreshold 44, the intermediate frequency component of the frequencysignal is ignored.

During the period t₃ to t₄ further analysis as will be described belowis carried out on the high frequency and low frequency components of thesignal to determine if the interference is from a know spurious source,namely, a mobile phone. In this case the interference is determined asbeing from a mobile phone, and because of this the low frequency energyalarm threshold 40 is set at an intermediate threshold value 40′″ attime t₄.

Before describing the setting of the low frequency energy alarmthreshold to the intermediate threshold value 40′″, the determination ofthe interference being that of a mobile phone or otherwise will first bedescribed. As mentioned above, it has been discovered that a mobilephone has low frequency and high frequency components, and the ratio ofthe energy of the high frequency component of the mobile phoneinterference to the energy of the low frequency component exceeds acertain predetermined ratio value. The predetermined ratio value dependson the algorithm used for determining the energy in the high and lowfrequency components of the interference signal, however, in thisembodiment of the invention the predetermined minimum value is two.Accordingly, in order to determine if the interference is that of amobile phone, the ratio of the energy of the high frequency component tothe energy of the low frequency component is determined, and if greaterthan two, the interference is deemed to be from a mobile phone.Otherwise, the interference is deemed to have resulted from passiveintermodulation or another unidentifiable source. As can be seen fromwaveforms D and F, the ratio of the energy of the high frequencycomponent to the energy of the low frequency component of the signalfrom time t₃ to t₅ is greater than two, and thus, the interference isdetermined to be mobile phone interference.

The intermediate threshold value 40′″ of the low frequency energy alarmthreshold is determined such that the low frequency energy of the mobilephone interference will not exceed the low frequency energy alarmthreshold when set at the intermediate threshold value 40′″, therebyavoiding false alarms. In this embodiment of the invention theintermediate threshold value 40′″ is derived from the energy of the highfrequency component of the mobile phone interference by multiplying theenergy of the high frequency component by an appropriate fractionalvalue stored in the microprocessor 28. The fractional value is dependenton the method used in determining the energies of the high frequency andlow frequency components of the frequency signal.

Thereby, with the low frequency energy threshold 40 set at theintermediate threshold value 40′″ between times t₄ and t₅ slow movementcan be detected. If slow movement occurred in the vehicle the energy ofthe low frequency component of the signal would exceed the intermediatethreshold value 40′″ of the low frequency alarm threshold 40. This wouldindicate the possible presence of an alarm condition, and depending onother checks carried out by the software in the microprocessor 28, aswill be described below, an alarm signal would be outputted on theoutput 29 from the microprocessor 28 to the alarm circuit 35.

At time t₅ the mobile phone interference disappears, and only backgroundnoise is present in the three components of the signal. Thus, from timet₅ the low frequency energy alarm threshold 40 is progressivelydecremented in steps until it is again at the first threshold value ofapproximately ten percent above the energy of background noise, and ismaintained at this value until the next disturbance is detected.

At time t₆ further disturbance is detected in the three components ofthe signal, and the energy of the high frequency component of the signalexceeds the high frequency energy threshold 44, thus indicatinginterference. Again the low frequency energy alarm threshold 40 isincreased to the maximum threshold value 40″. The ratio of the energy ofthe high frequency component to the energy of the low frequencycomponent of the signal is determined to ascertain if the interferenceis from a mobile phone. However, in this case the interference isdetermined as not being from a mobile phone, and more than likely is theresult of passive modulation interference. Since it is not possible todistinguish either a slow or a fast movement in the vehicle from suchinterference, the low frequency energy alarm threshold 40 is left set atthe maximum threshold values 40″ while the interference is present inorder to avoid false alarms, and the intermediate frequency component ofthe signal is ignored.

At time t₇ when the interference has ceased, the low frequency energyalarm threshold 40 is progressively decremented in steps similarly asdescribed with reference to the time period from t₅ to t₆. However, atany time while the low frequency energy alarm threshold is beingdecremented, if a disturbance is detected in either of the voltagewaveforms A or B of the low and intermediate frequency components of thesignal, and if the energy of either the low or the intermediatefrequency components of the signal exceeds the corresponding low orintermediate thresholds 40 or 42, and if the energy of the highfrequency component is not exceeding the high frequency energy threshold44, an alarm condition may be determined as having been detected, andfurther checks as will be described below are carried out. However, ifduring decrementing of the low frequency energy alarm threshold, theenergy of the high frequency component of the signal exceeds the highfrequency energy threshold 44, indicating the presence of interference,the low frequency energy alarm threshold 40 is set to the maximumthreshold value 40″, and would subsequently be reduced to theintermediate threshold value 40′″, if the interference was determined tobe mobile phone interference.

Although a trace of the signals corresponding to detection of a fastmovement in the absence of interference has not been illustrated in FIG.2, if a fast movement were detected in the absence of interference, thevoltage signal of the intermediate frequency component outputted by thebandpass filter 27, namely, the waveform B would exceed the upper andlower amplitude thresholds 32 and 33. Additionally, the energy of theintermediate frequency component of the signal illustrated in waveform Ewould exceed the intermediate frequency energy threshold 42, anddepending on the speed of the movement, the voltage in the low frequencycomponent of the signal, namely, waveform A may also exceed the upperand lower amplitude thresholds 30 and 31, and the energy in the lowfrequency component of the frequency signal, namely, waveform D may alsoexceed the low frequency energy alarm threshold 40, particularly if thelow frequency energy alarm threshold 40 is set at the first thresholdvalue 40′. Whether the movement signal appears in the low frequency orthe high frequency component of the signal depends on the speed of themovement. The slower the movement, the greater will be the energy in thelow frequency component of the frequency signal, and the faster themovement, the greater will be the energy in the intermediate frequencycomponent of the signal. Where the speed of the movement is halfwaybetween a very slow movement and a very fast movement, the energies inthe respective low and intermediate frequency components of the signalsmay be substantially similar, and the disturbance caused by the movementwill appear in both the low frequency and the intermediate frequencycomponents of the frequency signal.

The operation of the microprocessor 28 under the control of the softwarewill now be described with reference to the flow charts of FIGS. 3 to 5.As discussed above, the microprocessor 28 samples the outputs of thethree filters, namely, the low pass filter 25, the bandpass filter 27,and the high pass filter 26, respectively, at 2.04 millisecond intervalsduring each cycle period of 500 milliseconds. Thus, during each 500millisecond cycle period, two hundred and forty-four samples of theoutputs from each of the three filters 25, 26 and 27 are taken toconstruct the waveforms A, B and C. Additionally, as the outputs of thefilters 25, 26 and 27 are being sampled the energies of the low,intermediate and high frequency components of the signal are computedusing the algorithms which have already been described in order toconstruct the waveforms D, E and F of FIG. 2.

Referring initially to FIG. 3, the flow chart of FIG. 3 illustrates themain software routine under which the microprocessor 28 operates inorder to determine if a valid alarm condition has been detected. Theflow chart of FIG. 3 illustrates one cycle in which the respectiveoutputs of the three filters 25, 26 and 27 are sampled once, asdiscussed above during a cycle period the software goes through theroutine of FIG. 3 two hundred and forty-four times. Block 50 commencesthe routine, and the routine moves to block 51 which initialises themicroprocessor 28, and sets counters and stored values from the previouscycle period to zero. The routine then moves to block 52. Block 52checks if it is time for the next sample, and if not, waits until thenext sample is due to be taken, at which stage the routine moves toblock 53. Block 53 samples the voltages of the outputs of the filters25, 26 and 27, which are similar to the waveforms A, B and C of the lowfrequency, intermediate frequency and high frequency components of thesignal. Block 54 applies the algorithms for determining the energy ofthe respective low, intermediate and high frequency components of thesignal for computing the energy waveforms D, E and F. After the energyof the low, intermediate and high frequency components of the signalhave been determined by block 54, the routine moves to block 55 whichchecks if a cycle period has been completed. If not, the routine movesto block 56 which stores the largest value so far computed of energy forthe low frequency, intermediate frequency and high frequency componentsof the signal.

The routine then moves to block 57 which checks if the just computedenergy value of the intermediate frequency component of the signalexceeds the intermediate frequency energy threshold 42. If so, theroutine moves to block 58 which increments a count of the number oftimes during the cycle period the energy of the intermediate frequencycomponent of the signal exceeds the intermediate frequency energythreshold 42. The routine then moves to block 59. If block 57 determinesthat the energy just computed of the intermediate frequency component ofthe signal is equal to or less than the intermediate frequency energythreshold 42, block 57 moves the routine to block 59.

Block 59 checks if the high frequency energy value just computed exceedsthe high frequency energy threshold 44, and if so, the routine moves toblock 60 which increments the count of the number of times the energy ofthe high frequency component of the signal exceeds the high frequencyenergy threshold 44 during the cycle period, and the routine then movesto block 61. If block 59 determines that the just computed energy valueof the high frequency component of the signal is equal to or less thanthe high frequency energy threshold 44, the routine is moved to block61.

Block 61 checks if the just sampled voltage of the low frequencycomponent of the signal, namely waveform A, exceeds the upper or lowerthresholds 30 and 31. If so, the routine moves to block 62 whichincrements a count of the number of times the voltage of the lowfrequency component of the signal exceeds the upper and lower thresholds30 and 31 during a cycle period. The routine then moves to block 64. Ifblock 61 determines that the voltage of the low frequency component ofthe signal, namely, the waveform A is within the upper and lowerthresholds 30 and 31, the routine moves to block 64.

Block 64 checks if the just sampled value of the voltage of theintermediate frequency component of the signal, namely, waveform Bexceeds the upper or lower thresholds 32 or 33, and if so, the routinemoves to block 65. Block 65 increments a count of the number of timesthe voltage of the intermediate frequency component of the signalexceeded the upper and lower thresholds 32 and 33 during the cycleperiod. The routine then returns to block 52 and waits until it is timeto take the next samples. If block 64 determines that the voltage of theintermediate frequency component of the signal was within the upper andlower voltage thresholds 32 and 33, the routine is returned to block 52.

During each return to block 52 a counter (not shown) increments thenumber of passes of the routine until the two hundred and forty-fourthpass has been made, which ends a cycle period of 500 milliseconds. Onthe two hundred and forty-fourth pass having been made, the counter isreset to zero.

If block 55 had determined that the cycle period of 500 milliseconds hadbeen completed, in other words, the routine had gone through two hundredand forty-four passes, the routine is moved to block 70. Block 70 callsup a subroutine which selects the mode in which the anti-theft system 1is to operate during the next cycle period. The routine then moves toblock 71 which calls up a subroutine which resets the low frequencyenergy alarm threshold 40 for the next cycle period. This subroutine isdescribed below. The routine then moves to block 72 which calls up asubroutine to check if an alarm condition has been determined. Thissubroutine is also described below. After block 72 the routine moves toblock 73 which checks if the subroutine called up by block 72 determinedthat an alarm condition has been detected. If so, the routine moves toblock 74 which outputs an alarm signal to the alarm circuit 35 on theoutput 29 from the microprocessor 28. Otherwise, the routine moves toblock 75 which resets the counters to zero and the stored value storedduring the just completed cycle period to zero, and the routine is thenreturned to block 52 to wait until it is time to take the next samples.

Turning now to FIG. 4, a flow chart of the subroutine called up by block72 to check if an alarm condition has been determined is illustrated.The subroutine of FIG. 4 starts with block 80 and moves to block 81.Block 81 checks the mode in which the anti-theft system 1 is operating.The modes in which the anti-theft system 1 can operate will be describedin more detail below. Block 81 checks if the mode in which theanti-theft system is operating is a NO_INTERFERENCE mode. ANO_INTERFERENCE mode is a mode whereby no interference is detected andthe low frequency energy alarm threshold 40 is set at the firstthreshold value 40′, in other words, at approximately 10% above theenergy of the background noise in the low frequency component of thesignal. As illustrated in FIG. 2 the anti-theft system is operating inthe NO_INTERFERENCE mode during the time period t₀ to t₁ when nointerference is detected, and the low frequency energy alarm threshold40 is set at the first threshold value 40′. If the block 81 determinesthat the anti-theft system is operating in the NO_INTERFERENCE mode, thesubroutine moves to block 82, and checks the count stored by block 62 ofthe routine of FIG. 3 of the number of times the voltage of the lowfrequency component of the signal, namely, waveform A exceeded the upperand lower voltage thresholds 30 and 31 during the just completed cycleperiod. If the count is greater than or equal to a first predeterminedcount which typically is in the range of two to eight, the subroutinemoves to block 83 which checks if the maximum value of the energy of thelow frequency component of the signal stored by block 56 of the routineof FIG. 3 during the just completed cycle period exceeded the set lowfrequency energy alarm threshold 40. If so, the subroutine moves toblock 84 which indicates that an alarm condition has been determined.Since block 83 is only accessed when the anti-theft system is operatingin a NO_INTERFERENCE mode, the threshold value of the low frequencyenergy alarm threshold 40 against which the stored maximum value of theenergy of the low frequency component of the signal is compared, is thefirst threshold value 40′, which is the value of the low frequencyenergy alarm threshold when set at approximately 10% above thebackground noise energy.

If, on the other hand, block 82 determines that the count of the numberof times the voltage of the low frequency component of the signal storedby block 62 of the routine of FIG. 3 was less than the firstpredetermined count, or if block 83 determines that the maximum value ofenergy of the low frequency component of the signal stored by block 56of the routine of FIG. 3 was less than or equal to the low frequencyenergy alarm threshold 40, the subroutine moves to block 87. Block 87checks if the count of the number of times the energy of theintermediate frequency component of the signal exceeded the intermediatefrequency energy threshold stored by block 58 of the subroutine of FIG.3 exceeds a second predetermined count, which typically lies in therange two to eight, during the just completed cycle period. If so, thesubroutine moves to block 88 which checks if the count of the number oftimes the voltage of the intermediate frequency component of the signalexceeded the upper and lower amplitude thresholds 32 and 33 stored byblock 65 of the routine of FIG. 3 is greater than or equal to a thirdpredetermined count, which typically lies between two and eight, duringthe just completed cycle period. If so, the subroutine moves to block 84which determines that an alarm condition has been detected. If block 87determines that the count is less than or equal to the secondpredetermined count, or if block 88 determines that the count is lessthan the third predetermined count, then the subroutine is moved toblock 89.

Block 89 checks if the count of the number of times the voltage of thelow frequency component of the signal stored by block 62 of the routineof FIG. 3 during the just completed cycle period is greater than orequal to one, and if so, the subroutine moves to block 90. Block 90checks if the maximum value of the energy of the low frequency componentof the signal stored by block 56 of the routine of FIG. 3 during thejust completed cycle period exceeded the set low frequency energy alarmthreshold 40, and if so, the subroutine moves to block 84 whichdetermines that an alarm condition has been detected. If block 89determined that during the just completed cycle period the voltage ofthe low frequency component of the signal did not exceed the voltageupper and lower amplitude thresholds 30 and 31, the subroutine is movedto block 92 which determines that an alarm condition has not beendetected. Similarly, if block 90 determines that the maximum value ofthe energy of the low frequency component of the signal stored duringthe just completed cycle period is equal to or less than the set lowfrequency energy alarm threshold 40, the subroutine is moved to block92.

On the other hand, if block 81 determines that the anti-theft system 1was not in a NO_INTERFERENCE mode, the subroutine is moved to block 93.Block 93 checks if the anti-theft system 1 is in an INTERFERENCE mode.The INTERFERENCE mode is a mode when it is determined that theanti-theft system 1 is operating in the presence of interference. In theINTERFERENCE mode the low frequency energy alarm threshold 40 is set tothe maximum threshold value, in other words, to the value 40″ as, forexample, between time t₃ and t₄ of FIG. 2. In other words, when it ispossible that the interference is mobile phone interference. If block 93determines that the anti-theft system is in the INTERFERENCE mode, sinceit is not possible to detect a slow movement in this mode, and since inthis mode the intermediate frequency component of the signal is ignored,the subroutine is moved to block 92 which determines that an alarmcondition has not been detected. This is due to the fact that because ofthe level of interference, an alarm condition cannot be detected withoutthe possibility of the alarm being a false alarm. If block 93 determinesthat the anti-theft system is not in the INTERFERENCE mode, thesubroutine moves to block 94 which checks if the subroutine is in a PIMmode. This is a mode in which interference is caused by a signal from aspurious source which cannot be identified, for example, by passivemodulation. This is the mode in which the anti-theft system is operatingduring the period from time t₆ to t₇ of FIG. 2. In other words, it isthe time period during which the low frequency energy alarm threshold 40is set at the maximum value, namely, 40″. If block 94 determines thatthe anti-theft system 1 is operating in the PIM mode, the subroutinemoves to block 92 which determines that an alarm condition has not beendetected. As already described, this is because an alarm conditioncannot be detected without the possibility of the alarm condition beinga false alarm. If block 94 determines that the mode in which theanti-theft system is operating is not the PIM mode, the subroutine movesto block 95 which checks if the maximum value of the energy of the lowfrequency component of the signal stored by block 56 of the routine ofFIG. 3 during the just completed cycle period exceeded the set lowfrequency energy alarm threshold 40, and if so, the subroutine moves toblock 96 which determines that an alarm condition has been detected.Otherwise, the subroutine moves to block 92 which determines that noalarm condition has been detected.

After blocks 84, 92 and 96, the subroutine moves to block 97 which endsthe subroutine, and returns control of the microprocessor 28 to theroutine of FIG. 3, which then moves to block 73, which has already beendescribed.

The modes in which the anti-theft system 1 can operate will now bedescribed. In total the anti-theft system 1 can operate in seven modes.The seven modes are as follows:

-   1. NO_INTERFERENCE mode, this is the mode in which the anti-theft    system 1 operates when no interference is present. The anti-theft    system 1 is operating in the NO_INTERFERENCE mode between time to    and t, in FIG. 2. In this mode the low frequency energy alarm    threshold 40 is set to its first threshold value 40′.-   2. INTERFERENCE mode, this is the mode in which the anti-theft    system 1 is operating when interference is present, and the    interference cannot be identified as mobile phone interference, and    is such as to prevent a determination of an alarm condition being    made without the risk of a false alarm. The anti-theft system 1 is    operating in the INTERFERENCE mode during the time period t₆ to t₇    of FIG. 2. In the INTERFERENCE mode the low frequency energy alarm    threshold 40 is set at its maximum threshold value 40″.-   3. WAS_INTERFERENCE mode, the anti-theft system 1 is set to operate    in the WAS_INTERFERENCE mode when the interference which caused the    anti-theft system 1 to be operated in the INTERFERENCE mode has    ceased. The anti-theft system 1 is not illustrated operating in the    WAS_INTERFERENCE mode, however, operation of the anti-theft system 1    in the WAS_INTERFERENCE mode would be similar to the operation of    the anti-theft system 1 during the time period from t₇ onwards in    FIG. 2. During the WAS_INTERFERENCE mode the low frequency energy    alarm threshold 40 is progressively decremented from the maximum    threshold value 40″ in decremental steps. In the event of no further    interference being detected during the WAS_INTERFERENCE mode, the    low frequency energy alarm threshold 40 is decremented to the first    threshold value 40′.-   4. MOBILE_PHONE mode, in this mode the interference has been    determined as being that caused by a strong mobile phone signal. The    anti-theft system 1 is operating in the MOBILE_PHONE mode during the    period t₄ to t₅ of FIG. 2. In this mode the low frequency energy    alarm threshold 40 is set to the intermediate threshold value 40′″.-   5. WAS_MOBILE_PHONE mode, the anti-theft system 1 is set to operate    in the WAS_MOBILE_PHONE mode when the interference caused by the    strong mobile phone signal which caused the anti-theft system 1 to    be operated in the MOBILE_PHONE mode has ceased. The anti-theft    system 1 is operating in the WAS_MOBILE_PHONE mode during the time    period from t₅ to t₆ of FIG. 2. During the WAS_MOBILE_PHONE mode the    low frequency energy alarm threshold 40 is progressively decremented    from the intermediate threshold value in decremental steps. In the    event of no further interference being detected during the    WAS_MOBILE_PHONE mode, the low frequency energy alarm threshold 40    is decremented to the first threshold value 40′.-   6. PIM mode, this mode is similar to the INTERFERENCE mode, however,    the anti-theft system 1 is operated in the PIM mode when the    interference cannot be identified as being mobile phone    interference. The anti-theft system 1 is operating in the PIM mode    during the time period t₆ to t₇. In the PIM mode the low frequency    energy alarm threshold 40 is set at the maximum threshold value 40″.-   7. WAS_PIM mode, the anti-theft system 1 is set to operate in the    WAS_PIM mode when the interference which caused the anti-theft    system 1 to operate in the PIM mode has ceased. The anti-theft    system 1 is operating in the WAS_PIM mode during the time period    from t₇ onwards in FIG. 2. During the WAS_PIM mode the low frequency    energy alarm threshold 40 is progressively decremented from the    maximum threshold value 40″ in decremental steps. In the event of no    further interference being detected during the WAS_PIM mode, the low    frequency energy alarm threshold 40 is decremented to the first    threshold value 40′.

Referring now to FIG. 5, a flow chart of the subroutine called up byblock 71 of the routine of FIG. 3 on completion of each cycle period forresetting the low frequency energy alarm threshold 40 is illustrated.Block 100 starts the subroutine, and the subroutine moves to block 101which checks if the mode selected by block 70 of the routine of FIG. 3is the INTERFERENCE mode. If so, the subroutine moves to block 102 whichsets the low frequency energy alarm threshold 40 at the maximumthreshold value, namely, the value 40″. The subroutine then moves toblock 103 which terminates the subroutine, and hands back control of themicroprocessor 28 to the routine of FIG. 3, which then moves to block72. If block 101 determines that the mode selected by block 70 is notthe INTERFERENCE mode, the subroutine moves to block 105 which checks ifthe mode selected by block 70 is the PIM mode. If so, the subroutinealso moves to block 102 which as already described, sets the lowfrequency energy alarm threshold at the maximum threshold value. Thus,when the INTERFERENCE mode and the PIM modes are selected, the lowfrequency energy alarm threshold 40 is set to the maximum thresholdvalue in order to avoid false alarms.

If block 105 determines that the mode selected by block 70 is not thePIM mode, the subroutine moves to block 106. Block 106 checks if themode selected by block 70 is the WAS_INTERFERENCE mode, and if so, movesto block 107 which reduces the low frequency energy alarm threshold 40by one decremental step, until the low frequency energy alarm thresholdis reduced to the first threshold value, and the subroutine moves toblock 103 which as already described, hands control of themicroprocessor 28 back to the routine of FIG. 3 which proceeds to block72.

If block 106 determines that the mode selected by block 70 was not theWAS_INTERFERENCE mode, the subroutine moves to block 109 which checks ifthe mode selected by block 70 is the WAS_MOBILE_PHONE mode, and if so,the subroutine moves to block 107 which has already been described. Ifblock 109 determines that the mode selected by block 70 is not theWAS_MOBILE_PHONE MODE, the subroutine moves to block 111 which checks ifthe mode selected by block 70 is the WAS_PIM mode. If so, the subroutinemoves to block 107 which has already been described. Otherwise, thesubroutine moves to block 112.

Block 112 checks if the mode selected by block 70 is the MOBILE_PHONEmode. If so, the subroutine moves to block 114 which sets the lowfrequency energy alarm threshold 40 to the intermediate threshold value40′″. If block 112 determines that the mode selected by block 70 is notthe MOBILE_PHONE mode, the subroutine moves to block 116. In which case,the only remaining mode in which the anti-theft system 1 can be operatedis the NO_INTERFERENCE mode. Thus, block 116 calls up a subroutine forsetting and maintaining the low frequency energy alarm threshold 40 atthe first threshold value, which is set and maintained at a value ofapproximately 100% above the average energy of the background noise inthe low frequency energy component of the signal. This is carried out bymonitoring the energy of the background noise over a predeterminednumber of cycle periods and averaging the noise energy, and thendetermining 110% of the average noise energy, and setting the lowfrequency energy alarm threshold 40 at that value. Thus, as thebackground noise energy in the low frequency component of the signalvaries, the first threshold value 40′ the low frequency energy alarmthreshold 40 is likewise varied. Similarly, block 116 sets and maintainsthe intermediate frequency energy threshold 42 at a value ofapproximately 110% of the average energy of the background noise in theintermediate frequency component of the signal. Block 116 also sets andmaintains the high frequency energy threshold 44 at a value ofapproximately 110% of the average energy of the background noise in thehigh frequency component of the signal. Block 116 also maintains theupper and lower amplitude thresholds 30, 31, 32 and 33 of the voltage ofthe low frequency and intermediate frequency components of the signal at10% above and below the background noise values of the respectivevoltage signals. The subroutine then moves to block 103 which as alreadydescribed, returns control of the microprocessor 28 to the routine ofFIG. 3 at block 72.

Accordingly, by virtue of the fact that the microprocessor 28 under thecontrol of the software can identify interference caused by a mobilephone, and then sets the low frequency energy alarm threshold at theintermediate threshold value which is above the energy of the lowfrequency component of the mobile phone signal, false alarms are avoidedduring the presence of a mobile phone signal, and furthermore, and mostimportantly, the anti-theft system 1 continues to operate to detect slowmovements in the vehicle indicative of an unauthorised attempt to gainentry to or to enter the vehicle, and in the event of detecting suchmovement, an alarm signal is outputted for activating an alarm and/or aradio transmitter for transmitting a signal to a central monitoringstation or elsewhere alerting to the alarm condition. Monitoring for analarm condition is only suspended during interference, the source ofwhich cannot be determined as being a mobile phone, such as passiveintermodulation caused by coins or small metallic articles beingvibrated within the vehicle, or other such similar type interference,and thus false alarms are avoided.

While the method and apparatus for determining if movement has beendetected in an area under surveillance by a movement detector and fordistinguishing a signal resulting from movement from a signal resultingfrom a spurious source has been described for use in an anti-theftsystem for a motor vehicle, the method and the apparatus may be used fordetecting movement in any area under surveillance, and fordistinguishing a movement signal from a signal resulting from a spurioussource, for example, interference caused by a mobile phone, it isenvisaged that signals resulting from interference from other spurioussources may also be determined.

Additionally, while a specific movement detector circuit has beendescribed with reference to FIG. 1, it will be readily appreciated bythose skilled in the art that the method according to the invention fordetermining if movement has been detected in an area under surveillanceand distinguishing a signal resulting from a spurious source may be usedfor analysing a signal received from any other suitable type of detectorcircuit. Indeed, it is envisaged that the method may be used inconjunction with any of the detector circuits described in U.S. Pat. No.5,966,090 of McEwan, or indeed, any other suitable detector circuit.

While the anti-theft system has been described for use in a motorvehicle, the anti-theft system may be used for monitoring for movementin any area under surveillance, and the area being monitored would bedetermined by the burst widths of the radio frequency signals.

Additionally, while the various signals have been described as being ofspecific frequencies, other frequencies may be used without departingfrom the scope of the invention.

It will be appreciated that while the anti-theft system has beendescribed as operating the microprocessor under the control of softwaresuch that both a low frequency component and an intermediate frequencycomponent of the frequency signal is analysed for detecting anddifferentiating between fast and slow movements, it will be appreciatedthat in certain cases, only low frequency components may be analysed. Itwill also be appreciated that the frequency range of the low frequencycomponents, the intermediate frequency components and the high frequencycomponents may be varied.

It is also envisaged that the analysis of the high frequency and lowfrequency components of an interference signal resulting from mobilephone interference may be analysed for the purpose of determiningwhether the mobile phone signal is a strong signal or a weak signal.This will be determined by comparing the ratio of the energy of the highfrequency component to the energy of the low frequency component of themobile phone interference signal with two predetermined ratio values.One of the predetermined ratio values would be set so that energy ratiosover that predetermined ratio value would be indicative of a strongmobile phone signal, while the other predetermined ratio would be setsuch that ratios over that predetermined ratio value would be indicativeof weak mobile phone signals. In which case, the intermediate value ofthe low frequency energy alarm threshold would be derived from theenergy of the high frequency component of the mobile phone interferencesignal using two fractional values, one for a strong signal, and one fora weak signal.

1. A method for analysing a frequency signal from a movement detectorfor determining if movement has been detected in an area undersurveillance by the movement detector, and for distinguishing a signalresulting from movement from a signal resulting from a spurious source,characterised in that the method comprises the steps of: determining avalue of a characteristic of a low frequency component of the frequencysignal, comparing the determined value of the characteristic of the lowfrequency component with a set threshold value of a low frequency alarmthreshold for determining if the determined value of the characteristicof the low frequency component is indicative of movement having beendetected, determining a value of a characteristic of a high frequencycomponent of the frequency signal to ascertain if the value of thecharacteristic of the high frequency component is indicative of a signalfrom a known spurious source having been detected, and setting the lowfrequency alarm threshold at an intermediate threshold value, if thedetermined value of the characteristic of the high frequency componentis indicative of a signal from a known spurious source, the intermediatethreshold value being greater than the value of a correspondingcharacteristic of a low frequency component of the frequency signalresulting from the signal from the known spurious source for preventinga false alarm.
 2. A method as claimed in claim 1 in which the methodfurther comprises outputting an alarm signal in response to thedetermined value of the characteristic of the low frequency component ofthe frequency signal exceeding the low frequency alarm threshold whenthe low frequency alarm threshold is set at the intermediate thresholdvalue.
 3. A method as claimed in claim 1 in which an alarm signal isoutputted in response to the determined value of the characteristic ofthe low frequency component of the frequency signal exceeding the lowfrequency alarm threshold when the low frequency alarm threshold is setat a first threshold value, and when the determined characteristic ofthe high frequency component of the frequency signal does not exceed ahigh frequency threshold, the first threshold value of the low frequencyalarm threshold being less than the intermediate threshold valuethereof.
 4. A method as claimed in claim 1 in which the first thresholdvalue of the low frequency alarm threshold is set at a value above thebackground noise level in the low frequency component of the frequencysignal in the absence of a spurious signal.
 5. A method as claimed inclaim 1 in which the determined value of the characteristic of the highfrequency component of the frequency signal is compared with thecorresponding determined characteristic of the low frequency componentof the frequency signal for determining if the frequency signal hasresulted from a signal from a known spurious source.
 6. A method asclaimed in claim 1 in which the set threshold value of the low frequencyalarm threshold is reduced when a signal from a spurious source isdetermined as having ceased.
 7. A method as claimed in claim 1 in whichthe characteristic, the value of which is determined in respect of thelow frequency component of the frequency signal is a characteristicindicative of the energy of the low frequency component of the frequencysignal, and the low frequency alarm threshold is an energy threshold. 8.A method as claimed in claim 1 in which the characteristic, the value ofwhich is determined in respect of the high frequency component of thefrequency signal is a characteristic indicative of the energy of thehigh frequency component of the frequency signal, and the high frequencythreshold is an energy threshold.
 9. A method as claimed in claim 1 inwhich the frequency signal is passed through a low pass filter forproviding the low frequency component of the frequency signal, and thefrequency signal is passed through a high pass filter for providing thehigh frequency component of the frequency signal, and the frequencysignal is passed through a bandpass filter for providing theintermediate frequency component of the frequency signal.
 10. A methodas claimed in claim 1 in which the frequency signal is derived from amicrowave movement detector.
 11. Apparatus for analysing a frequencysignal from a movement detector for determining if movement has beendetected in an area under surveillance by the movement detector, and fordistinguishing a signal resulting from movement from a signal resultingfrom a spurious source, characterised in that the apparatus comprises: ameans for determining a value of a characteristic of a low frequencycomponent of the frequency signal, a first comparing means for comparingthe determined value of the characteristic of the low frequencycomponent with a set threshold value of a low frequency alarm thresholdfor determining if the determined value of the characteristic of the lowfrequency component is indicative of movement having been detected, ameans for determining a value of a characteristic of a high frequencycomponent of the frequency signal to ascertain if the value of thecharacteristic of the high frequency component is indicative of a signalfrom a known spurious source having been detected, and a means forsetting the low frequency alarm threshold at an intermediate thresholdvalue, if the determined value of the characteristic of the highfrequency component is indicative of a signal from a known spurioussource, the intermediate threshold value being greater than the value ofa corresponding characteristic of a low frequency component of thefrequency signal resulting from the signal from the known spurioussource for preventing a false alarm.
 12. Apparatus as claimed in claim11 in which a means is provided for outputting an alarm signal, themeans for outputting the alarm signal being responsive to the determinedvalue of the characteristic of the low frequency component of thefrequency signal exceeding the low frequency alarm threshold when thelow frequency alarm threshold is set at the intermediate thresholdvalue.
 13. Apparatus as claimed in claim 12 in which the means foroutputting the alarm signal is responsive to the determined value of thecharacteristic of the low frequency component of the frequency signalexceeding the low frequency alarm threshold when the low frequency alarmthreshold is set at a first threshold value, and when the determinedcharacteristic of the high frequency component of the frequency signaldoes not exceed a high frequency threshold, the first threshold value ofthe low frequency alarm threshold being less than the intermediatethreshold value thereof.
 14. Apparatus as claimed in claim 11 in whichthe means for setting the threshold value of the low frequency alarmthreshold sets the low frequency alarm threshold at the first thresholdvalue, which is a value above the background noise level in the lowfrequency component of the frequency signal, in the absence of spurioussignals.
 15. Apparatus as claimed in claim 11 in which a third comparingmeans is provided for comparing the determined value of thecharacteristic of the high frequency component of the frequency signalwith the corresponding determined characteristic of the low frequencycomponent of the frequency signal for determining if the frequencysignal has resulted from a signal from a known spurious source. 16.Apparatus as claimed in claim 11 in which the means for setting thethreshold value of the low frequency alarm threshold reduces the lowfrequency alarm threshold in response to a signal from a spurious sourcehaving ceased.
 17. Apparatus as claimed in claim 11 in which the meansfor determining the value of the characteristic which is determined inrespect of the low frequency component of the frequency signaldetermines the value of a characteristic indicative of the energy of thelow frequency component of the frequency signal, and the low frequencyalarm threshold is an energy threshold.
 18. Apparatus as claimed inclaim 11 in which the means for determining the value of thecharacteristic which is determined in respect of the high frequencycomponent of the frequency signal determines the value of acharacteristic indicative of the energy of the high frequency componentof the frequency signal, and the high frequency threshold is an energythreshold.
 19. Apparatus as claimed in claim 11 in which a low passfilter is provided for filtering the frequency signal for producing thelow frequency component of the frequency signal, and a high pass filteris provided for filtering the frequency signal for producing the highfrequency component of the frequency signal, and a bandpass filter isprovided for filtering the frequency signal for producing theintermediate frequency component of the frequency signal.
 20. Apparatusas claimed in claim 11 in which a microwave movement detector isprovided for providing the frequency signal.
 21. An anti-theft systemcomprising a transmitter circuit for transmitting a sequence of burstsof electromagnetic energy to produce a sensor field, the transmittedbursts having burst widths which vary according to a pattern, a receivercircuit for receiving a combination of the transmitted bursts andreflections of the transmitted bursts and for producing a combinedoutput frequency signal, and apparatus as claimed in claim 11 forreceiving and analysing the frequency signal.
 22. A method as claimed inclaim 3 in which the determined value of the characteristic of the highfrequency component of the frequency signal is compared with the highfrequency threshold for determining if the value of the characteristicof the high frequency component is indicative of a signal from aspurious source, and if the determined value of the characteristic ofthe high frequency component exceeds the high frequency threshold, thelow frequency alarm threshold is set at a maximum threshold value, themaximum threshold value of the low frequency alarm threshold beinggreater than the intermediate threshold value thereof for preventingfalse alarms.
 23. A method as claimed in claim 22 in which the maximumthreshold value of the low frequency alarm threshold is greater than thegreatest value of the characteristic of the low frequency component offrequency signals likely to be encountered.
 24. A method as claimed inclaim 1 in which the frequency range of the low frequency component ofthe frequency signal is selected for determining slow movements withinthe area under surveillance.
 25. A method as claimed in claim 1 in whichthe method further comprises the step of determining the value of acharacteristic of an intermediate frequency component of the frequencysignal, and comparing the determined value of the characteristic of theintermediate frequency component with an intermediate frequencythreshold for determining if the determined value of the characteristicof the intermediate frequency component of the frequency signal isindicative of a fast movement having been detected.
 26. A method asclaimed in claim 25 in which the low, high and intermediate frequencycomponents of the frequency signal are sampled at predeterminedintervals during a predetermined cycle period, and preferably, themaximum value of the characteristic determined during each cycle periodfor each frequency component is recorded, and the alarm signal isoutputted in response to the recorded maximum values.
 27. A method asclaimed in claim 25 in which the intermediate frequency range of thefrequency signal is in the range of 40 Hz to 100 Hz, the low frequencyrange of the frequency signal is in the range up to 100 Hz, and the highfrequency range of the frequency signal exceeds 100 Hz.
 28. A method asclaimed in claim 1 in which the characteristic, the value of which isdetermined in respect of the low frequency component of the frequencysignal is a characteristic indicative of the amplitude of the lowfrequency component of the frequency signal, and the low frequency alarmthreshold is an amplitude threshold.
 29. A method as claimed in claim 25in which the characteristic, the value of which is determined in respectof the intermediate frequency component of the frequency signal is acharacteristic indicative of the energy of the intermediate frequencycomponent of the frequency signal, and the intermediate frequencythreshold is an energy threshold.
 30. A method as claimed in claim 26 inwhich the number of times the determined value of the characteristicindicative of the amplitude of the low frequency component of thefrequency signal exceeds the set amplitude threshold value of the lowfrequency alarm threshold is recorded during each cycle period, and thealarm signal is outputted in response to the recorded number of timesthe determined value of the characteristic indicative of the amplitudeexceeds the set amplitude threshold value of the low frequency alarmthreshold.
 31. Apparatus as claimed in claim 13 in which a secondcomparing means is provided for comparing the determined value of thecharacteristic of the high frequency component of the frequency signalwith the high frequency threshold for determining if the value of thecharacteristic of the high frequency component is indicative of a signalfrom a spurious source, and if the determined value of thecharacteristic of the high frequency component exceeds the highfrequency threshold, the means for setting the threshold value of thelow frequency alarm threshold sets the low frequency alarm threshold ata maximum threshold value, the maximum threshold value of the lowfrequency alarm threshold being greater than the intermediate thresholdvalue thereof for preventing false alarms.
 32. Apparatus as claimed inclaim 11 in which a means for determining the value of a characteristicof an intermediate frequency component of the frequency signal isprovided, and a fourth comparing means is provided for comparing thedetermined value of the characteristic of the intermediate frequencycomponent with an intermediate frequency threshold for determining ifthe determined value of the characteristic of the intermediate frequencycomponent of the frequency signal is indicative of fast movement havingbeen detected.
 33. Apparatus as claimed in claim 32 in which a samplingmeans is provided for sampling the low, high and intermediate frequencycomponents of the frequency signal at predetermined intervals during apredetermined cycle period, and a storing means is provided for storingthe maximum value of the characteristic determined during each cycleperiod for each frequency component, and the means for outputting thealarm signal is responsive to the stored maximum values.
 34. Apparatusas claimed in claim 11 in which the means for determining the value ofthe characteristic which is determined in respect of the low frequencycomponent of the frequency signal determines the value of acharacteristic indicative of the amplitude of the low frequencycomponent of the frequency signal, and the low frequency alarm thresholdis an amplitude threshold.
 35. Apparatus as claimed in claim 32 in whichthe means for determining the value of the characteristic which isdetermined in respect of the intermediate frequency component of thefrequency signal determines the value of a characteristic indicative ofthe energy of the intermediate frequency component of the frequencysignal, and the intermediate frequency threshold is an energy threshold.36. Apparatus as claimed in claim 32 in which the means for determiningthe value of the characteristic which is determined in respect of theintermediate frequency component of the frequency signal determines acharacteristic indicative of the amplitude of the intermediate frequencycomponent of the frequency signal, and the intermediate frequencythreshold is an amplitude threshold.